1176
a-Lactams. VI.'
Photochemistry of a-Lactams'
John C. Sheehan3 and M. Mehdi Nafissi-V. Contribution from the Department of Chemistry, Massachusetts Institutz of Technology, Cambridge, Mussachusctts 02139. Receiwd June 21, 1968 Abstract: Three a-lactams were found to possess a distinct absorption in the ultraviolet 250 mK (E -102 1. mol-' cm-I), which exhibits a hypsochromic shift in going from n-hexane to ethanol as solvents. This type of absorption is characteristic of n -+. K* excitation. Irradiation of a pentane solution of 1,3-di-t-butyIaziridinone (I), l-t-butyl-3-(1-methylcyclopenty1)aziridinone(II),and 1-t-butyl-3-(1-methylcyclohexyl)aziridinone(111) in a quartz vessel by a uv light source produced carbon monoxide and the corresponding Shiff bases as the main products. This fragmentation formally resembles Norrish type I photodecarbonylation of amides, although the yields from a-lactams are strikingly higher. A radical mechanism on the basis of this similarity is suggested. Neither the uv spectra nor the photolysis of a-lactams have been reported previously.
T
he recent synthesis4 of 1,3-di-t-butylaziridinone (I) demonstrated that low stabillty and high chemical reactivity are not inherent properties of three-membered cyclic 1actams.j Compound I decomposes slowly at 170" and uncatalyzed solvolysis requires a n elevated temperature and prolonged reaction time. The stability of a-lactams is, however. profoundly influenced by the nature of substituents on nitrogen and C-3. Thus it wits noted4,,>that the absence of hydrogen atoms on the carbon directly attached t o C-3 of the ring results in a marked increase in thermal stability, since one mode of decomposition, namely eliminative isomerization t o an open chain unsaturated amide, is blocked. This suggestion received further confirmation by the recently reported synthesis of several stable a-lactam^.^-^ In a continuing investigation of a-lactanis reactions 1-t-butyl-3-( 1 -methylcyclopentyl)aziridinone (11) and I-t-butyl-3-(l-methylcyclohexyl)aziridinone (111) were synthesized and subjected t o photolysis.
H
Synthesis The synthesis of I has been reported previously. a-Lactams 11 and 111 were synthesized from the corresponding cyclic ketones according to Scheme I. Both a-lactams show n band at 1835 c n - ' i n the infrared, characteristic for aziridinone, j with no N -Htype absorption. The nmr (CCl,) spectra possess a singlet, 1 H, at 164 H z which is distinctive for C(3)-H in a-lactams. Photolysis of a-Lactams a-Lactams I, 11, and 111 manifest a well-defined maximum in the ultraviolet at about 250 nip, which shows a hypsochromic shift in going from whexane t o ethanol as solvents (see Table I). a characteristic of n-x* transitions. The data in Table I are significant in that the n-x* absorption of cyclic1" and open-chain amides lies below 220 mp, which has been attributed to the interaction of lone-pair electrons on nitrogen with carbonyl K bonds." In a-lactanis. hoRever, the amide resonance is sterically inhibited and form B is highly strained. Therefore, one nlight expect that
RJyf N
I I, R = C(CH&
Is, R = m C H 3 III,R=
OCH3
(1) Part V : J. C. Shcehan and J. 14. Beesen, J . A m . Chem. Soc., 89, 366 (1967). (2) This work was aided by a grant from the U. S . Public Health Service, National Institutes of Health (No. C-2239). (3) T o whom inquiries concerning this paper should be addressed. (4) J. C. Sheehan and J. H . Beeson, J . A m . Chem. Soc., 89, 362 (1967). (5) F o r the synthesis and reactions of a-lactams sec: 1. Lengyel and J. C. Sheehan, Angew. Chrm., 80, 27 (1968); also Angew. Chern. Intern. Ed. Engl., 7, 2 5 (1968). (6) K. Bott, Aiigew. Chern., 79, 943 (1967); Aiigew. Chem. Inrerri. Ed. Engl., 6 , 946 (1967). (7) E. R. Talaty, A . E. Dupuy, Jr., and A . E. Cancienne, Jr., J . Heterocj*c[icChcni., 4, 657, (1967). (2) H. E. Bauingarten, R . D. Clark, L. S . Endres, L. D . Hagemeier, and V. J. Elia, Terrahedron Letters, 5033 (1967). (9) E. R . Talaty and A . E. Dupuy, Jr., Chem. Coniniuti., 790 (1968).
A
B
a-lactams would exhibit the n-r* absorption band at nearly the same position as their cyclic ketone analogs, for example, cyclopropanones.12,13 The fact that the n-r* band in a-lactamsappears600-1000 AI4lower than in cyclopropanones may indicate that the interaction of lone-pair electrons on nitrogen with the carbonyl xorbital electrons is still significant. (IO) 7 - and &lactams show n-r' absorption a t 208 mw ( e 3 X IO2 I. mol-] cm-1): Y .V. Moiseev, G. I . Balyukov, and M . I . Vinnik, Zh. Fir. Khint., 37, 570 (1963). (11) S . F. Mason, Q u a r f .Rec. (London), 15, 287 (1961). (12) Si. J. Turro and W. B. Hammond, J . A m . Cheni. Soc., 88, 3672 (1966). (13) J. F. Pasoz and F. D. Greene, ibid., 89, 1030 (1967). (14) These numbers a r e based 011 the data from ref 12 and 13.
Jourriirl of'the American Chemical Society ,! 91:5 ,! February 26, 1969
1177 Scheme I
n (HC) C=O
2o
np3 ( H C ) C\
1.CH,Br, Mg t
'0 OH
(CZHJzO 2. NH,CI, H,O
1.CH,-CCI,
nC,,CH,
BF3-H'04t(H,Qn
U
2. H,O
CHz LOzH
IVa, b Va, b
a,n=4 b,n=5
Table I. The n-a* Band in the Uv Spectra of a-Lactams
R A Y 0
N
-A,, !?-Hexane
I
Bu.1
C(CHd8
mw (log Ethanol
251 (2.03)
240 (2.19)
246 (1 .77)
233 (2.17)
e"' ocH'
+-------
Methanol
234 (1.98)
247 (2.16)
sium hydroxide.
amides.'j The main products of a-lactam reaction are carbon monoxide and the corresponding imines. This fragmentation is in sharp contrast with the thermal decomposition of these compounds, 1,16 which gives a carbonyl compound (aldehyde of ketone) and an isocyanide.
R'
When a pentane solution of 1,3-di-t-butylaziridinone (I) in a quartz vessel was exposed to the radiation from a Hanovia 679 A 36 lamp for 24 hr complete decomposition occurred. No significant change was observed in the dark or in Pyrex glass, which is not transparent to light with wavelength shorter than 280 mp, even in the presence of benzophenone. Carbon monoxide and N-t-butylneopentylidenimine (VIII), identical with a sample synthesized by an independent method, were isolated in 97 % yield. The irradiation of 1-t-butyl-3-(l-methylcyclopentyl)aziridinone (11) and 1-t-butyl-3-( 1-methylcyclohexyl)aziridinone (111) under the same conditions afforded similar products, namely, carbon monoxide (84-90 %) and the corresponding imine. The complete decomposition of I1 and 111, however, requires more than 24
H
R
@ N
I
Bu-t
b,pentane, 203, >, 24 h r
+
RCH=NBu-t
bo N
A
I
R"
A radical mechanism similar to photoinduced decarbonylation of amides'5 can be formulated for the decomposition of a-lactams by uv light, although other mechanisms (e.g., ionic or concerted) cannot be excluded on the basis of present evidence.
,(fo R'
-----f hv
N
I
CO
VIII-x VIII, R = t-Bu Ix, R =
The infrared and nmr spectra of imine
X were consistent with the assigned structure. The photoinduced decomposition of a-lactams resembles Norrish type I photodecomposition of
An intermediate of the type
0(""
x, R = O
C
H
3
has been suggested for the photorearrangement of certain anilide~.'~" hr. The structure of imine IX was established by comparison with an authentic sample obtained from the conde,nsation of 1-(1-methylcyclopentane)carboxaldehyde and t-butylamine in the presence of potas-
(15) G. H. Booth and R. G. W. Norrish, J . Chem. Soc., 188 (1952). (16) J. C. Sheehan and I. Lengyel, J . A m . Chem. Soc., 86, 746 (1964). (17) (a) M. Fischer, Tetrahedron Letters, 4295 (1968); (b) G. Chavanne and L. de Vogel, Bull. SOC.Chem. Belges, 37, 141 (1928).
Sheehan, Najissi-V.
Photochemistry of a-Lactams
1178 142-143'); n Z 5 D1.4562 1 z Z 5 ~1.4560); infrared (neat) 2700 and 1715 cm-l. Melting points were determined on a Kofler hot-stage micro1-(1-Methylcyc1opentane)carboxaldehyde (1 g) was condensed scope. The infrared spectra were recorded on a Perkin-Elmer with t-butylamine ( 5 ml) in the presence of potassium hydroxide 237 spectrophotometer, and nmr spectra were obtained on a Varian (2 g) in 15 hr to give 0.9 g (70%) of JX: bp 156-157"; infrared A-60 and/or T-60 spectrometer. Refractive indices were deter(neat) 2960, 2875, and 1665 cm-l; nmr (CC14) 448 (singlet, 1 H), mined by a Zeiss Opton Abbe refractometer. Ultraviolet spectra 98 (multiplet, 8 H), 66 (singlet, 9 H), and 63 Hz (singlet, 3 H). were recorded on a Cary 14 spectrophotometer. Microanalyses Preparation of l-r-Butyl-3-(l-methyIcyclopentyl)aziridinone (11). were performed by Dr. S. M. Nagy and his associates at MassaN-r-Btityl-2-bromo-2-(l-methylcyclopentyl)acetamide(VIIa) was chusetts Institute of Technology. All melting points and boiling converted to aziridinone I1 by the general method described in ref points are uncorrected. 4 in 77% yield: bp 72-73" (0.4 mm); infrared (neat) 1835 cm-l; Preparation of N-t-Butyl-2-(l-methyIcyclopentyl)-2-bromoacet- iimr (CClJ 164 (singlet, 1 H), 98 (multiplet, 8 H), 88 (singlet, 9 H), amide (VIIa). 1-Methylcyclopentan-1-01, mp 36", bp 140' (lit.17b and 64 Hz (singlet, 3 H); uv, XhEc,":'246 mp (log e: 1.770). mp 36", bp 135-1 36"), was converted to 1-methylcyclopentylacetic A n d . Calcd for Ci2H2,NO: C, 73.7; H! 10.83; N, 7.172. acid by the method described by BottI8 in 33% yield: bp 112-113" Found: C: 73.93; H. 10.98; N, 7.36. (11 mm) (1it.I8 131-132" (20 mm)); 1 2 2 5 ~1.4554; infrared (neat) Preparation of l-r-But~I-3-(l-methyIcyclohexyl)aziridinone(111). 1705 cm-l; nmr (CCI,) 739 (singlet, 1 H), 140 (singlet, 2 H), 96 N-r-Butyl-2-(1-methylcycloliexyl)-2-bromoacetamide(VIIb) was (multiplet, 8 H), and 67 Hz (singlet, 3 H). This acid was converted converted to aziridinone TIT by the general method described in ref to the acid chloride by thionyl chloride in amost quantitative yield: 4 in 90.5% yield: bp 81.2" (0.35 mm); infrared (neat) 1835 cm-l; ~ infrared (neat) 1800 cm-'. The bp 63-65' (10 mm); n Z 91.4608; nmr (CCI:) 164 (singlet, 1 H), 95 (broad, 10 H), 84 (singlet, 9 H), acid chloride was brominated in refluxing carbon tetrachloride, 247 mp (log c 2.16). and 67 Hz (singlet, 3 H); uv, A'":,' ~ . ~was D and the a-bromo acid chloride, bp 62" (0.5 mm), ~ I ~ 1.5551 A M / . Calcd for Ci.,HyaNO: C, 74.64; H, 11 ; N, 6.7. Found: treated with t-butylamine in methylene chloride at -20". VIIa C, 73.71; H, 11.06; N,6.8. was isolated with an over-all yield of 72%: mp 98-99"; infrared Photolysis of 1,3-Di-r-butylaziridinone (I). A solution of 1,3(KBr disk) 3310, 1663, and 1555 cm-1; nmr(CC1,) 378 (broad, 1 H), di-r-butylaziriciinone (I),* 0.521 g, in pentane (25 ml) contained 252 (singlet, 1 H), 101 (multiplet, 8 H), 91 (singlet, 9 H), and 67 Hz i n a quartz test tube was placed in a water bath at a distance of 5 (singlet, 3 H). cm from the radiation source. After attachment of a gas collecAnal. Calcd for CI2H2?BrNO: C, 52.36; H , 8.0; N, 5.09; tion bulb, the solution was irradiated by a Hanovia 679 A 36 (450 Br, 28.7. Found: C, 52.70; H, 8.46; N, 4.81; Br, 27.94. W) lamp for 23 hr at 20'. After filtration, tlie solvent was evapPreparation of N-t-Butyl-2-(l-methyIcyclohexyl)-2-bromoacetorated at 0' tinder reduced pressure. The residue, which was a amide (VIIb). 1-Methylcyclohexan-1-01, bp 170" (lit.I9 168"), was colorless liquid, bp 120"; infrared (neat) 1670 cm-l; nmr (CCI,) converted to 1-methylcyclohexylacetic acid according to the method 437 (singlet, 1 H). 63 (singlet. 9 H), and 59 Hz (singlet, 9 H), was described by Bottis in 45% yield: bp 104-105" (1.3 mm) (lit.is identical with the synthetic N-r-butylneopentyiidenimine. bp 148-150" (20 mm)); 1 2 2 5 , 5 ~1.4673; infrared (neat) 1710 cm-'; In addition to this liquid, 85 nil of gas was collected. A 67-nil nmr (CCI,) 730 (singlet, 1 H), 132 (singlet, 2 H): 84 (broad, 10 H), portion of this gas was absorbed by an acidic solution of cuprous and 62 Hz (singlet, 3 H). This acid was converted to acid chloride chloride (a reagent for carbon monoxide).24 This represents a D by thionyl chloride in 97% yield: bp 49" (0.6 mm); / I ~ ~1.4723; Iield of 97:; of carbon monoxide. A minor quantity of solid infrared (neat) 1800 cm-1. The acid chloride was brominated in polymer: the structure of which was not determined, was also seprefluxing carbon tetrachloride and the a-bromo acid was treated arated from tlie reaction mixture. with r-butylamine in methylene chloride at -20'. VIIb was isoPhotolysis of l-r-Buty1-3-(l-meth~lcyclopentyl)aziridinone (11). lated in an over-all yield of 84%; mp 124-125'; infrared (KBr A solution of 0.67 g of I1 in pentane (25 ml) contained in a quartz disk) 3305, 1665, and 1557 cm-l; nmr (CCI,) 366 (broad, 1 H), test tube was irradiated by uv light as described previously. Imine 250 (singlet, 1 H), 88 (broad, 10 H), 80 (singlet, 9 H), and 64 Hz IX and carbon monoxide were the only products detected; the (singlet, 3 H). yield was 90% based on carbon monoxide. Anal. Calcd for C1,H2,BrNO: C, 53.79; H, 8.33; N, 4.82: Photolysis of l-r-Butyl-3-(l-methylcyclohe~yl)aziridinone (111). Br,27.53. Found: C,54; H, 8.23; N, 5; Br,27.55. A solution of l-r-but~l-3-(l-methylcyclohexyl)aziridinone(III), Preparation of IV-r-Butyl-l-(l-meth~lcyclopentgl)methylidenim- 0.5 g, in pentane (25 ml) contained in a quartz test tube was irine (IX). 1-(1-Methylcyclopentyl)carbinol, bp 75.6" (6 mm), radiated by LIV light as described before; 45 ml (corrected volume) ~ 2 1.4592 5 ~ (lit.20bp 168'; n Z o1.4586) ~ (2.28 g), was converted to of carbon monoxide was collected which represents 84% yield. 1-(1-methylcyc1opentane)carboxaldehyde by the method described The liquid was filtered and the solvent was evaporated at 0'. A by Holum.21 The yield was 1.85 g (82%): bp 141-142" (lit.22 colorless liquid was obtained: infrared (neat) 1675 cm-l (C=N); nmr (CCI,) 448 (singlet, 1 H), 96 (multiplet, 10 H), 67 (singlet, 9 H), and 63 Hz (singlet, 3 H).
Experimental Section
(18) (19) (20) (21) (22)
I
